Sunday, 25 September 2011

Pseudo sixth nerve palsies

Level 1: We had a very engaging grand round from our neuro-ophthalmologist, Nadeem Ali, on Thursday.  He presented a case of convergence spasm. This reminded me of a case I saw when I was a trainee when I made an incorrect diagnosis of bilateral 6th nerve palsies, when the patient actually had convergence spasms.

The clue to this diagnosis is that when the try and ask the patient to abduct the eye they can't and the eye movements are associated with a rather bizarre jerky nystagmus; the nystagmus is not regular and the jerks occur at a lower frequency. The clue to the diagnosis is that the pupils are noted to constrict and the vision for distant objects will blur. Convergence spasms occurs as a result of the activation of the accommodation or triple reflex; (1) adduction of the eyes (medial recti), constriction of the pupil (constrictor pupillae)  and accommodation (ciliary muscle). As a result of the accommodation reflex the visual acuity for distant objects changes, i.e. in abduction the patient develops a myopia and needs a concave lens (negative dioptres) to correct the vision.

Although convergence spasms is usually not due to "organic disease" and work-up is negative (Sarkies and Sanders, 1985), its response to botox has led some to consider it a form of dystonia (Kaczmarek, 2009). However, convergence spasms can be associated with organic pathology (Guiloff et al., 1980), for example trauma, multiple sclerosis and other brainstem pathology.

Please don't assume that convergence spasms is due to "hysteria" or is "functional", this is what is printed in most text books.

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